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| United States Patent |
6,063,651
|
|
Edelstein
, et al.
|
May 16, 2000
|
Method for activating fusible links on a circuit substrate
Abstract
A method and apparatus for activating fusible links on a circuit substrate.
The circuit substrate is supported in a fixture which is cooled to a below
ambient temperature. Cooling of the circuit substrate decreases the
absorption of energy by the substrate, permitting a smaller spot size
laser beam having a lower wavelength to be employed for interrupting the
fusible links. The substrate is cooled by a refrigeration coil in heat
transfer with the fixture holding the substrate. Moisture formation is
avoided by placing the substrate and laser source in a controlled
atmosphere.
| Inventors:
|
Edelstein; Daniel Charles (New Rochelle, NY);
Narayan; Chandrasekhar (Hopewell Junction, NY)
|
| Assignee:
|
International Business Machines Corporation (Armonk, NY)
|
| Appl. No.:
|
018145 |
| Filed:
|
February 3, 1998 |
| Current U.S. Class: |
438/132; 257/E23.15; 438/799 |
| Intern'l Class: |
H01L 021/82 |
| Field of Search: |
438/132,467,463,799
|
References Cited
U.S. Patent Documents
| 4795720 | Jan., 1989 | Kawanabe et al.
| |
| 5089443 | Feb., 1992 | Kerey et al.
| |
| 5374590 | Dec., 1994 | Batdorf et al.
| |
| 5420455 | May., 1995 | Gilmour et al.
| |
| 5523253 | Jun., 1996 | Gilmour et al.
| |
| 5585663 | Dec., 1996 | Bezama et al.
| |
| 5608257 | Mar., 1997 | Lee et al.
| |
| 5622892 | Apr., 1997 | Bezama et al.
| |
| 5625218 | Apr., 1997 | Yamadera et al.
| |
| 5838361 | Nov., 1998 | Corbett | 347/262.
|
Primary Examiner: Bowers; Charles
Assistant Examiner: Pert; Evan
Attorney, Agent or Firm: Pollock, Vande Sande & Amernick
Claims
What is claimed is:
1. A method for activating fusible links on a circuit substrate without
damaging the substrate comprising:
cooling said substrate with a refrigerant in thermal contact with a holder
of said substrate to a temperature which is below an ambient temperature
to decrease the energy absorption characteristics of said substrate; and
melting each fusible link when said substrate has been cooled and energy
absorption has been lowered whereby said fusible links are interrupted
without damaging said substrate.
2. The method for activating fusible links according to claim 1 wherein
each fusible link is melted using laser radiation.
3. The method according to claim 2 wherein said laser radiation has a
wavelength of less than 1.3 .mu.m.
4. The method according to claim 2 further comprising subjecting said
substrate to a reactive gas during said step of melting each fusible link.
5. A method for activating fusible links on a substrate comprising:
disposing said substrate in a controlled atmosphere;
radiating said fusible links with laser radiation to melt each of said
links; and
cooling said substrate during melting of said links with a circulating
refrigerant in thermal contact with said substrate to reduce the
absorption of said radiation by said substrate to prevent said substrate
from being damaged by said laser radiation.
6. The method according to claim 5 wherein said controlled atmosphere
contains a reactive gas.
7. The method according to claim 6 wherein said laser radiation wavelength
is less than 1.3 microns.
8. The method according to claim 1 wherein said substrate is cooled to a
temperature as low as 198.degree. K.
9. The method according to claim 1 wherein said substrate is cooled to a
temperature as low as 77.degree. K.
Description
BACKGROUND OF THE INVENTION
The present invention relates to an apparatus and method for activating
electrically programmable fusible links in VLSI circuits, specifically
fusible links having a reduced interlink spacing, without damaging the
underlying substrate.
High density dynamic random access memories (DRAMS) are designed with
memory cell redundancy. The redundant memory cells avoid the loss of an
entire memory in the event that a minor number of memory cells are not
functioning. Activation of the redundant memory cells is accomplished by
fusible links which are strategically placed throughout the memory.
Activation of a fusible link results in the disabling of the defective
memory cell, while enabling in its place a redundant memory cell.
The process of "blowing" fusible links is implemented by heating the
fusible link which is to be blown. The heated fusible link melts or
evaporates, creating an open circuit for replacing the defective memory
cells with a functional cell.
The fusible links are made of aluminum, copper and other high conductive
metal or metal alloys. The conductive fusible link generally has a central
width portion which is smaller than the ends, to reduce the amount of
energy necessary to melt the fusible link to create an open circuit
condition.
The melting of fusible links may be accomplished using a laser beam having
a controlled beam width. Additionally, the fusible links may be opened by
applying a high current thereto, heating the fusible links as a result of
power dissipation in the fusible link which is sufficient to melt the
fusible links.
Utilizing the laser for blowing the fusible links requires that two
conditions be observed to avoid damage to the circuit. The first is to
have a beam width which is sufficiently narrow to blow only a single
fusible link without inadvertently blowing an adjacent fusible link. The
second condition which must be observed is avoiding damage to the
underlying silicon substrate which supports the fuse elements.
These requirements are at odds with each other in that while it is
desirable to decrease the distance between fusible links, i.e., pitch,
there is a corresponding reduction in the laser beam width which produces
a high energy beam which may damage the silicon substrate. In order to
further increase the density of DRAMS, it is therefore desirable to reduce
the pitch between fusible links, while at the same time being able to blow
the fusible links without damaging the underlying silicon surface or
adjacent fusible links.
SUMMARY OF THE INVENTION
It is an object of this invention to provide a method and apparatus for
activating fusible links without damaging the underlying circuit
substrate.
It is a further object of this invention to provide a method and apparatus
for blowing fusible links having a reduced link pitch without damaging the
underlying circuit substrate.
These and other objects of the invention are provided by a method and
apparatus in accordance with the invention. A method is provided which
will melt a fusible link having a reduced pitch without damaging the
underlying silicon substrate. The process for blowing the fusible link
reduces the laser light absorption characteristics of the substrate. By
reducing the absorption characteristics of the substrate, higher energy
laser beams (with smaller wavelength) may be used, having a
correspondingly smaller beam diameter for safely blowing fusible links on
a tight pitch. The decreased beam width of the laser beam permits a
decrease in the fusible link pitch and an increase in the laser beam
energy density.
In accordance with a preferred embodiment, a substrate bearing the fusible
links is supported in a chuck or fixture which is cooled to a temperature
below ambient. As the cooling temperature decreases, the absorption
characteristics of the substrate changes. As a result, it is possible to
reduce the wavelength of the laser beam and corresponding beam width,
i.e., spot size, to less than 1 .mu.m. The substrate fixture may be cooled
through a refrigeration unit having cooling coils in heat transfer contact
with the fixture which in turn reduces the substrate temperature.
DESCRIPTION OF THE FIGURES
FIG. 1 illustrates a series of fusible links on a semiconductor substrate;
FIG. 2 illustrates the energy absorption curve for a silicon substrate as a
function of incident radiation; and
FIG. 3 illustrates the general configuration of an apparatus for blowing
fusible links.
DESCRIPTION OF THE PREFERRED EMBODIMENT
Referring now to FIG. 1, the general configuration of a plurality of
fusible links on a VSLI silicon substrate 100 is shown. The fusible links
have a width 104, and are separated from adjacent fusible link 108 by a
spacing dimension 103. Each of the fusible links are connected to
circuitry on the silicon substrate 100 for activating redundant memory
cell elements or, in other applications, such as trimming the value of a
component.
The blowing of a fusible link is typically accomplished with a laser beam
having a wavelength of approximately 1.047 .mu.m. The wavelength is chosen
to avoid damaging the underlying substrate 100 due to energy absorption by
the substrate. The laser of the aforementioned wavelength has a minimum
diameter, or spot size which dictates the minimum fuse pitch 103. The
minimum pitch size is necessary to avoid any inadvertent damage to an
adjacent fusible link 108 from a base beam having a spot size which is
large enough to overlap any neighboring fusible link. The damage zone 106
associated with the fusible link blowing process is directly related to
beam size. Thus, the fusible link pitch must be limited by the spot size
which is directly related to the wavelength of the incident laser
radiation.
A reduction in the laser operating wavelength to 1 .mu.m or less will
provide for a spot size in the order of 1 .mu.m, which permits fusible
link pitches of that order. The absorption of radiation by the silicon
substrate is strongly related to the wavelength, and increases
exponentially as the wavelength decreases.
Referring now to FIG. 2, an energy absorption curve for a silicon substrate
is shown as a function of incident radiation wavelength for two different
temperatures. As is evident from FIG. 2, the energy absorption (shown in
arbitrary units) on the y axis increases significantly as the wavelength
decreases. However, the magnitude of energy absorption decreases
significantly with temperature. Line 202 represents the absorption for a
constant temperature 298.degree. K., while line 204 demonstrates the
absorption of the silicon substrate at a constant temperature of
70.degree. K.
As the incident radiation wavelength decreases from 1 .mu.m to 0.5 .mu.m,
the absorption increases correspondingly as shown by line 202 for the
temperature of 298.degree. K. representing a room temperature condition.
It is evident from observing the absorption characteristics of silicon that
a decrease in absorption of several orders of magnitude is possible by
lowering the temperature of the substrate. Thus, where 0.5 .mu.m radiation
would have produced a value of absorption which would damage the silicon
substrate, the same substrate at a lower temperature would not suffer
damage from the same wavelength of radiation. As a preferred temperature,
reducing the substrate temperature to 198.degree. K. will provide
significant decreases in the energy absorption by the substrate. Using
liquid nitrogen as a coolant for cooling the substrate permits
temperatures as low as 77.degree. K. (the boiling point for liquid
nitrogen) to be achieved which substantially eliminates any substrate
damage.
In accordance with the invention, the lower temperature condition during
blowing of fusible links enables use of radiation wavelengths below 1
.mu.m, without suffering any substrate damage.
The consequences of lowering the temperature of the substrate provide for
benefits even in the presence of higher wavelength lasers, such as those
presently used having a wavelength of 1.047 .mu.m or greater. Presently,
this wavelength of radiation requires approximately 0.4 .mu.j of energy
within a 3.5 .mu.m diameter spot size to blow an aluminum based fusible
link. The same spot size can damage a silicon substrate if 0.65 .mu.j of
energy is applied to the substrate surface. Consequently, the laser beam
must be operated such as to produce a high enough energy level, i.e., 0.4
.mu.j to blow the fusible link, but maintain an energy level below 0.65
.mu.j to avoid damage to the silicon substrate.
The present invention, by cooling the substrate, permits the damage
threshold to be moved to 0.8 .mu.j, thus avoiding the consequences of a
narrow process window wherein energy levels produced by a laser must be
tightly controlled.
Referring now to FIG. 3, an apparatus for blowing fusible links on a
substrate 304 is illustrated. The enclosure 300 includes a substrate
holder 302, such as a chuck or other fixture for holding the substrate 304
in place. A focusing lens 310 is shown in association with a mirror 308
directing the laser beam from laser 306 to a focused spot on a fusible
link formed on the silicon substrate 304. Substrate 304 is cooled by a
cooling coil 502 contained within the fixture 302 in heat transfer
relationship with the substrate 304. The cooling coils are connected to a
conventional refrigeration unit 500 which may utilize liquid nitrogen as a
coolant, permitting coolant temperatures in the range of 77.degree. K. to
be obtained. A temperature sensor 501 embedded in the fixture 302 is used
to provide a temperature reading of the fixture and substrate to the
refrigeration unit 500.
An inlet 301 and outlet 302 to the chamber 300 permits the insertion of a
reactive gas such as chlorine into the chamber 300 providing a controlled
atmosphere for the substrate. The reactive gas reacts with the fusible
link to enhance the ability of the laser beam to blow the fusible link. In
the event the fusible link contains a passivation layer, the gas may also
interact with the passivation layer to enhance the laser beam's ability to
react and melt the fusible link.
The refrigeration unit 500 may use as a refrigerant liquid nitrogen, which
is circulated through the cooling coil 502 in the fixture 302. Temperature
sensor 501 is attached to the fixture which provides for temperature
sensing of the substrate. The thermostat on the refrigeration unit in
combination with a signal from the temperature sensor 501 establishes a
constant desirable temperature for the substrate.
The cooling of the substrate and supporting fixture may produce
condensation of moisture during melting of the fusible links by the laser
beam. A moisture free ambient atmosphere may be maintained by introducing
dry air to the inlet 301 and expelling the air from the outlet 302.
Although the foregoing technique is most advantageous when fusible links
are blown using a laser beam, the same benefits can be achieved for other
techniques for blowing the fusible links, such as for instance providing a
current through the fusible link which will heat and melt the link.
The foregoing description of the invention illustrates and describes the
present invention. Additionally, the disclosure shows and describes only
the preferred embodiments of the invention, but as aforementioned, it is
to be understood that the invention is capable of use in various other
combinations, modifications, and environments and is capable of changes or
modifications within the scope of the inventive concept as expressed
herein, commensurate with the above teachings, and/or the skill or
knowledge of the relevant art. The embodiments described hereinabove are
further intended to explain best modes known of practicing the invention
and to enable others skilled in the art to utilize the invention in such,
or other, embodiments and with the various modifications required by the
particular applications or uses of the invention. Accordingly, the
description is not intended to limit the invention to the form disclosed
herein. Also, it is intended that the appended claims be construed to
include alternative embodiments.
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